Spline structures are used to transmit torque between two components, particularly when a degree of relative movement is to be accommodated in a direction orthogonal to the direction of the torque.
Multi-plate devices are commonly used as brakes or clutches in transport and machinery, and as aircraft brakes. These devices rely on the application of an engagement force to press a series of friction discs together to transmit torque.
When used as a brake, such friction discs usually comprise intercalated sets, one set being rotor discs secured by splines for rotation with a rotor, whilst the other set are stator discs held by splines carried by a stator. The two sets of splines permit the discs to slide axially into and out of rubbing engagement. One or more actuators are carried by the stator and are arranged to press the outermost discs axially towards each other, thereby forcing all of the discs into mutual rubbing engagement so that braking torque is applied by the stator to the rotor. The stator is typically an axle or a torque tube linked with an axle integral with the landing gear, and the rotor is a wheel.
When used as a clutch, the overall arrangement is the same except that the stator is replaced by a second rotor so that engagement of the friction discs transmits torque between the two rotors.
As the friction discs are pressed together, the friction between them generates a significant amount of heat and the discs can reach a temperature of up to 3,0000° Centigrade in the case of a large aircraft breaking at full landing speed. This heat is initially absorbed by the disks which conduct the heat into the rotor and stator in the case of a brake, or into the two rotors in the case of a clutch. This heating is particularly intense in the splines which are also required to transfer the torque generated by the disks. The splines are consequently required to take very large forces in multi-point bending at high temperatures and their reliability is clearly safety critical. For this reason, the splines have to be formed from a high performance material. Very few materials are capable of achieving this performance, and typically high strength steels, stainless steels and nickel alloys are used. Although these special materials enable the required performance to be achieved, they are dense and add significant weight to an aeronautical brake system.
Throughout this specification and claims the term “metal” is used to include both metals and metal alloys, and named metals are used to include their alloys. Light metals, ceramics and polymer composites are being used increasingly and other transport and machinery to reduce weight and provide improved performance and efficiency. Titanium offers the great strength at high operational temperatures but insufficient strength at the very high temperatures developed in some applications, particularly aeronautical brake systems for heavy aircraft, unless the spline structures are made larger than the equivalent steel or nickel spline structures. Nickel provides greater strength at high operational temperatures but is a much heavier metal.
Heavy and high performance aircraft are often provided with multi-plate brakes of which the outer peripheries of the rotor friction discs engage the wheel via an annular series of individual splines known as drive bars. Each drive bar has its ends attached to the inside of the wheel rim by wheel attachment points at its ends. Two different types of attachment are commonly used, typically a radial bolt securing one end of the drive bar to the inside of the wheel rim, and a generally cylindrical pin extending longitudinally along an axis parallely spaced from the axis of rotation and engaging a corresponding aperture formed inside the wheel rim. In this manner each drive bar is securely connected to the wheel whilst being able to expand longitudinally and being readily removable for servicing or replacement.
If such drive bars are made from titanium, their transverse dimensions must be increased to provide the requisite strength, and this reduces the volume between the wheel rim and the axle splines with the result that the volume of the brake discs has to be decreased. Aircraft brake manufacturers are increasingly paid by the number of landings made on a set of brake discs, this being generally proportional to the volume of the discs. The selection of titanium for making drive bars has the advantage of significant weight reduction when compared with conventional steel or nickel alloys due its lower density. Although each drive bar is quite small, it should be borne in mind that each wheel can have many drive bars and large aircraft can have 20 wheels. In addition to the weight reduction achieved, the rotational energy of each wheel is also reduced enabling the brakes to decelerate the airframe more rapidly for a given braking force, or allowing a lower braking force to achieve the same deceleration.
It is an object of this invention to provide a reinforced drive bar that is of lower weight than conventional drive bars made of steel or nickel alloys, but does not significantly reduce the volume available for the friction discs. It is another object of this invention to provide reinforced spline structures from a range of metals for varying purposes.